Nitrogen pools and turnover in arable soils under different durations of organic farming: I: Pool sizes of total soil nitrogen, microbial biomass nitrogen, and potentially mineralizable nitrogen

Soil organic matter contents, soil microbial biomass, potentially mineraliz able nitrogen (N) and soil pH values were investigated in the Ap horizons o f 14 field plots at 3 sites which had been under organic farming over vario us periods. The objective was to test how these soil properties change with the duration of organic farming. Site effects were significant for pH valu es, microbial biomass C and N, and for potentially mineralizable N at 0-10 cin depth. The contents of total organic C, total soil N, and potentially m ineralizable N tended to be higher in soils after 41 versus 3 years of orga nic farming, but the differences were not significant. Microbial biomass C and N contents were higher after 41 years than after 3 years of organic fan ning at 0-10 cin depth, and the pH values were increased at 10-27 cm depth. Nine years of organic farming were insufficient to affect soil microbial b iomass significantly. Increased biomass N contents help improve N storage b y soil micro-organisms in soils under long-term organic farming.